In the prior art, the sashes of sliding sash windows were originally balanced by cast iron sash weights connected to cables passing over and around pulleys at the upper end of the window frame. Subsequently, these sash weights were replaced by single elongated sash balancing tension springs with their upper ends connected to cables which passed upward to and around pulleys at the tops of the window frames and then downward to connections to the upper ends of the sashes. The lower ends of the sash balancing tension springs were anchored to the lower ends of the metal or plastic channel sash guides in which the sashes were adapted to slide upward and downward while their weights were balanced by the forces applied to the cables as the tension springs became increasingly stretched. Such installations, however, have been disadvantageous in that the springs required stretching beyond their elastic limits to such excessive lengths that when each sash was in a closed position, its spring was excessively stretched to exert unnecessary balancing force to the sash whereas when the sash was fully open, the spring was so relaxed as to apply insufficient balancing force to the sash, with the result that the sash tended to sag downward so as not to be fully closed.
The present invention provides a sash balance which distributes the balancing force of a shorter stretching balancing spring between several sections of cables passing around multiple upper and lower pulleys, thereby reducing the excessive stretching of the springs to or beyond their elastic limits and preventing such sash sagging resulting from such excessive spring stretching and consequent excessive variation of the balancing forces of the prior art single pulley sash balancing spring installations.
It is an object of the present invention also to provide an extremely simple and essentially miniature structure for the balancing mechanism with all plastic molded parts and tiny pulleys which serve to provide smooth operating conditions and which are readily assembled and maintained.